to stimulate neurohypophysis, the hypothalamus sends nerve impulses which stimulate it to secrete oxytocin and ADH

*Does not synthesize any hormones - hormones from this gland are produced by neurosecretory cells in hypothalamus and secreted down axons w vesicles to be stored and later released from posterior pituitary

the nuclei in the hypothalamus which synthesize hormones (oxytocin and ADH) to be sent to the posterior pituitary

Specifically: paraventricular and

supraoptic nuclei

The axons from these nuclei make up hypothalamohypophyseal tract

oxytocin

hormone made by paraventricular and supraoptic nuclei in hypothalamus

targets uterus and breasts after delivery of baby

In uterus, stimulates contractions THROUGH POSITIVE FEEDBACK

In breasts, stimulates milk "let down" in response to infant sucking

*The purpose in men and non-pregnant women is unclear, but thought to promote feelings of sexual pleasure during and after intercourse and encourages emotional bonding btwn mating pair. So some call it the "love hormone"

vasopressin

ADH - antidiuretic hormone

hormone which decreases amount to urine the body produces

causes arterioles to constrict, thereby increasing blood pressure

targets ducts in kindey and sweat glands in skin to minimize water loss

osmoreceptors

found in the hypothalamus, monitor blood osmotic pressure (relates to concentration of blood)

What happens with high blood osmotic pressure

The high pressure (meaning high concentrated blood) stimulates hypothalamic osmoreceptors; Which in turn activate the neurosecretory cells that synthesize and release ADH

consists of approx 5000 amino acids, which more than 100 are the amino acid tyrosine

*"bin" - making Thyroid hormone from ingredients in the bin

C cells

also called parafollicular cells

a scattered group of cells surrounding each follicle

*on the outside of each follicle - so in betwn follicles

How is T3 & T4 made from TGB

to synthesize the thyroid hormones, TGB is released into the lumen of thyroid follicles

Within TGB, 1 or 2 iodine atoms attach to each tyrosine molecule

The tyrosine molecules then link to form thyroid hormone, containing either 3 or 4 atoms of iodine

So how are the thyroid hormones synthesized and put into circulation?

When needed, TGB moves into follicular cells, and digestive enzymes cleave T3 and T4 from the TGB molecule

T3 and T4 are lipid soluble, so they can diffuse through the plasma membrane and bind to the carrier protein thyroxin-binding globulin (TBG) in the blood

T4

thyroxin (tetraiodothyronine)

(tetra = 4)

T4, hormone secreted by thyroid

*prefix describes # of iodine molecules

T3

Triiodothyronine

*prefix describes # of iodine molecules

What stimulates the hypothalamus to secrete TRH & Whats the result

TRH = thyrotropin-releasing hormone

low T3 and T4 levels OR low metabolic rate stimulates release of TRH

this results in the anterior pituitary producing TSH (thyroid-stimulating hormone) which is released into the blood and binds to TSH receptors in follicular cells, stimulating secretion of T3 & T4 into blood

Elevated T3 inhibits release of TRH and TSH (negative feedback)

*Most of the T4 released is converted to the more potent T3

Required steps for T3 & T4 synthesis

Iodine trapping

Synthesis of TBG

Oxidation of iodide

Iodination of tyrosine

Coupling of T1 and T2 to make T3 and T4

Pinocytosis and digestion of colloid

Secretion of thyroid hormones

Transport of T3 and T4 in the blood

Iodine trapping

First required step in T3 & T4 synthesis

Iodine circulates in blood as iodide ( I- )

Iodide is actively transported into the follicular cells

Because of this process the thyroid gland contains most of the iodide in body

synthesis of TGB

TGB (thyroglobulin) is a glycoprotein produced by follicular cells

it contains large numbers of the amino acid tyrosine

Tyrosine is the site on the TGB molecule that will bind with iodine

Oxidation of iodide

Before iodide can bind to tyrosine, it must be oxidized and combine with another iodide to form an iodine molecule ( I2 )

Iodination of tyrosine

The side chain of tyrosine may pick up one (T1) or two (T2) iodine molecules

Coupling of T1 and T2 to make T3 and T4

As one of the last steps, two tyrosine molecules are joined to form either T3 (T1+ T2) or T4 ( T2 + T2)

Pinocytosis and digestion of colloid

Once synthesized, the iodine-containing TGB reenters the follicular cells and digestive enzymes break down the molecule, releasing the formed T3 and T4

Secretion of thyroid hormones

T3 and T4are lipid soluble, so they freely pass the cell membrane into the interstitial fluid and into the blood